Height sensor for hydrofoil watercraft

ABSTRACT

A method for detecting the relative position of a gaseous-liquid interface with respect to a datum point and an apparatus embodying the same for use in hydrofoil height sensors. A transmitter situated on the watercraft&#39;&#39;s hull directs an ultrasonic signal at the water surface. A portion of that signal is transmitted through the surface and refracted thereby to an ultrasonic receiver preferably situated on the hydrofoil. Evaluation of the transmission time of the ultrasonic signal from the transmitter to the receiver yields significant information concerning the relative position of the water surface with respect to either the hull or the hydrofoil. Factors for successful implementation of this technique are discussed in detail, and reference is made to a specific embodiment of a detector for transmission times.

[451 Nov. 28, 1972 [54] HEIGHT SENSOR FOR HYDROFOIL WATERCRAFT [72]Inventor: Charles P. Wright, Seattle, Wash.

[73] Assignee: The Boeing Company, Seattle,

Wash.

[22] Filed: April 20, 1970 [21] Appl. N0.: 30,049

V 52] US. Cl ..340/1 L, 73/290 V, 340/3 R, 340/5 R [51] Int. Cl ..G01s9/68 [58] Field of Search ..340/1 R, l L, 3 C, 3 R, 3 E, 340/5 R, 5 S;73/290 V Primary Examiner-Richard A. Farley Attorney-Christensen, Sanbom& Matthews ABSTRACT A method for detecting the relative position of agaseous-liquid interface with respect to a datum point and an apparatusembodying the same for use in hydrofoil height sensors. A transmittersituated on the watercrafts hull directs an ultrasonic signal at thewater surface. A portion of that signal is transmitted through thesurface and refracted thereby to an ultrasonic receiver preferablysituated on the hydrofoil. Evaluation of the transmission time of theultrasonic signal from the transmitter to the receiver yieldssignificant information concerning the relative position of the watersurface with respect to either the hull or the [56] References Citedhydrofoil. Factors for successful implementation of UNITED STATESPATENTS this technique are discussed in detail, and reference is made toa specific embodiment of a detector for trans- 3,080,752 3/1963 Rich..73/29O V mission times 2,960,678 11/1960 Beard et al ..340/1 R 9Clains, 5 Drawing Figures TRANSMISISIO/Y D/SPLA Y m rmmce T/ME ,6

DE 7' E C TOI? l SYSTEM SYSTEM PATENTEnlmv 28 m2 SHEET 2 OF 3 IN VENTOR.

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INVENTOR. CHARLES P h/F/6HT (Z/MJZZWdl H M/MM ATTORNEYS HEIGHT SENSORFOR I'IYDROFOIL WATERCRAFT BACKGROUND OF THE INVENTION This inventionrelates in general to an apparatus and a method for sensing the positionof an object relative to a gaseous-liquid interface, and, in particular,to an apparatus and a method for sensing the position of a submergedfoil of a hydrofoil watercraft relative to the water surface.

There has been in recent years an increasing use of watercraft which aresupported during travel by a foil which is in contact with the water.These foils generally comprise wing-like devices which produce a liftingforce when propelled through the water and are divided into two types.The first is designed for use in relatively smooth waters and ispartially submerged during travel. As is commonly known, watercraftusing foils of this type begin travelling with the hull thereof incontact with the water. When the craft reaches a predetermined speed,the lift exerted on its foils is sufficient to raise the hull above thewater. The depth of submergence of the foils is self-regulated and isdetermined by a balancing of the lift provided at the given propulsionspeed against the weight of the watercraft.

Because of their self-regulated operation, these foils are not suitablefor operation in the ocean or in other situations where extremely roughwater is to be encountered. In such cases, a second type of foil is usedwhich is totally submerged during any and all operating speeds. Thesefoils truly act as underwater wings. The height of the watercraft, andthus the depth of submergence of the foil, is varied by changing theangle of attack of the foil with respect to the direction of watercrafttravel.

Such watercraft invariably include automatic control systems formaintaining the depth of submergence of the foil at a predeterminedvalue. This value must be maintained because it is desirable to maintainthe lift of the foil relatively constant, and lift is greatly reduced ifthe foil ventures too close to the water surface. In addition, it isimperative that damage to the foil be avoided, such as might occur ifthe foil were to raise out of the water and thereafter to crash downthereupon. Finally, it is desirable to maintain the hull of thewatercraft a certain distance above the peaks of the on-coming waves toavoid contact therebetween and potential damage to the hull.

To maintain depth of submergence at a predetermined value, it isnecessary to know the actual depth of the foil at any instant of time. Anumber of sensors have been proposed and utilized to determine actualdepth. In one of these, a plurality of electrical contacts are spacedvertically along a strut supporting the foil from the hull of thewatercraft. Measurement of height is made by measuring the electricalresistance between the electrodes and a reference electrode which isalways disposed within the water. If the water level changes withrespect to the electrodes, the number of electrodes and thus themeasured electrical resistance varies proportionally. In a similardevice, a pair of rod electrodes are disposed on the strut. Thedielectric between these electrodes comprises air and water, therelative proportions thereof depending on the depth of submergence ofthe foil. By measuring the capacitance of the electrodes by suitablemeans, this depth can be ascertained.

Although these devices give somewhat accurate readings of foil depth,they are not particularly useful in ocean-going or rough-water hydrofoilcraft. Because the wave action in rough water is very complex, and ofteninvolves rapid changes in wave height, these devices, being mounteddirectly above the foil whose attitude is to be controlled, do notprovide sufficient time for system response.

For rough-water hydrofoil watercrafts, it has been common to use asensor which measures the height of the hull or the depth of the foilwith respect to the water by conventional sonar techniques. In a firstapproach, an ultrasonic transmitter is disposed on the prow of thewatercraft and directs its beam at the water surface. An ultrasonicreceiver in proximity thereto receives a portion of the ultrasonic wavethat is reflected from the water surface. If the velocity of the wavethrough the air medium is known, the height of the prow above the watersurface can be determined by noting the transmission time of the wavefrom the transmitter to the water surface and back to the receiver. In asecond approach, both the transmitter and receiver are disposedunderwater and measure foil depth by the transmission time of theultrasonic wave through the water medium. In both, a number of methodsfor detecting the transmission time are utilized, including pulsed andcontinuous wave signals.

By placing the transmitter and receiver on either the prow of the boator the leading edge of the foil and appropriately directing the beamfrom the transmitter, the height of any in-coming wave may be measuredin front of the watercraft and supplied to the automatic control systemfor the foils in sufficient time to anticipate a change in water leveladjacent the foil.

However, these approaches using sonar techniques have not proved toprovide reliable sensing. Since sonar requires that the wave bereflected from the water surface in order for a measurement oftransmission time to be made, its reliability is dependent on thecharacteristics of that surface. Due to the complex wave action inherentin any rough water, a substantial portion of the ultrasonic wavedirected at that surface by the ultrasonic transmitter is reflected indirections other than towards the ultrasonic receiver. In other words, alarge portion of the transmitted wave is dispersed, and only a verysmall portion thereof is reflected back to the receiver.

It can be immediately perceived that if a continuous wave system isbeing utilized, very high power levels are required to insure continuousreception of the reflected wave. In such cases, the actual transmittingelements must be capable of handling these high power levels. Where apulsed system is used, perhaps a less expensive and smaller transmittingelement can be utilized. However, the dispersing characteristic of thesurface dictates that some of the transmitted pulses will not bereflected back to the ultrasonic receiver. The result is an intermittentoperation of the sensor with a resultant intermittent operation of theautomatic control system.

Without reliable and continuous height or depth sensing in the open sea,for example, operation of the hydrofoil watercraft may become extremelydangerous.

It is therefore an object of this invention to provide an improvedheight or depth sensing technique for a hydrofoil watercraft.

It is another object of this invention to provide an improved height ordepth sensor for a hydrofoil watercraft which provides reliable,continuous and accurate measurement of the position of the water surfacerelative to the watercraft.

It is a broader object of this invention to provide a method for sensingthe position of a gaseous-liquid interface relative to a fixed datum.

SUMMARY OF THE INVENTION These objects and others are achieved, briefly,by directing an ultrasonic signal from the watercraft through the air tothe water surface, receiving .underwater the portion of that signal thatis refracted by passage through the water surface, andmeasuring thetransmission time of the signal between the transmitter and receiver.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF A PREFERRED EMBODIMENTWith reference to FIG. 1, a hydrofoil watercraft includes a hull andfirst and second struts 11, 12 which ducer should transmit theultrasonic signal uniformly throughout a predetermined beam so as toassist in reliable operation of the sensor for various slopes of thewater.

The ultrasonic signal produced bytransmitter 16 is directed at the watersurface 17. For example, the signal may travel along the path indicatedby ray 32a to surface 17, and thence along the path indicated by ray 32wto receiver 18. Included in receiver 18 is a means for detecting thepresence of the ultrasonic signal and producing an electrical signaltherefrom. If a continuous wave processing is used, the frequency andphase of the electrical signal is proportional tothat of the ultrasonicsignal. If a pulsed processing isus'ed, the electrical signal needindicate only the reception of the ul trasonic signal. Receiver 18 maythus comprise any underwater microphone, such as that known 'as a hydrophone. p

' The electrical signal from receiver 18 is'transmitted by a lead 19 toa transmission time detector 20 which also supplies an energizing signalto transmitter 16 by a lead 21. The function of detector 20 is todetermine the elapsed time of transmission of the ultrasonic signal areaffixed atone end thereof to hull l0 and which support at the other endthereof a hydrofoil 14. By means not shown, hydrofoil 14 is rotatable onstruts 11 and 12 to vary the angle of attack thereof relative to thewatercrafts direction of travel. The hydrofoil watercraft illustrated inFIG. 1 is designed for operation in rough water, such as is encounteredin the ocean, and thus hydrofoil 14 is intended to be totally submergedduring craft operation.

To maintain the hull 10 a predetermined distance above the water surface17, or the hydrofoil 14 a predetermined depth therebelow, this inventionuses an ultrasonic transmitter 16 disposed on the hull 10 and anultrasonic receiver 18 disposed on the craft below the water line,preferably on hydrofoil 14. The transmitter 18 may include a source ofultrasonic energy whose output comprises a continuous waveform or aseries of pulses, depending on the particular processing circuitry usedto detect the signal transmission time between the transmitter 16 andthe receiver 18. Also included in transmitter 16 is an ultrasonictransducer for producing an ultrasonic signal in the air surroundinghull 10. This transducer is connected to a source of electrical energywithin transmitter 16 and may comprise a ceramic transducer, apiezoelectric crystal, or the like which converts the electrical energyinto an ultrasonic signal suitable for transmission through the airmedium. As will be discussed in more detail hereinafter, this transfromthe transmitter 16 tothe receiver 18. As will be noted shortly, thiselapsed time is dependent on the position of the water surface 17 withrespectto transmitter l6 and receiver 18. Since the distance betweentransmitter 16 and receiver 18 is always fixed, this transmission timecan be thereby related to position of the water surface 17 with respectto either the hydrofoil 14 or hull 10.

In turn, transmission time detector 20 supplies an output signal to aninterface circuit 22 which in turn supplies output signals toanautomatic control system 24 and to a display means 26. The automaticcontrol system 24 may be any of those commonly known which operate on aninput signal corresponding to hydrofoil depth to provide appropriatecontrol signals to the propulsion and hydrofoil an'gle'control systemsof the watercraft. The display 26 may comprise an x-y recorder plottinghydrofoil depth or hull height against 7 time, an instantaneousdisplayincluding a cathode ray tube having both the instantaneous heightand an average height symbol displayed thereon, or the like.

Elements 20-26 have been used in previous depth sensors and hydrofoilcontrol systems and therefore individually form no part of thisinvention. Although an illustrative embodiment of the transmission timedetector 20 will be discussed later with reference to FIG. 5, thatembodiment is for purposes of explanation only and is not to beconstrued as a limitation upon this invention.

Rather, this invention rests upon the discovery that the time oftransmission of an ultrasonic signal along both an air path and a waterpath can be reliably and consistently determined for varyingconfigurations of the water surface. As has been already discussed, theprevious sonar methods and apparatus detected the transmission time ofan ultrasonic signal to and from the water surface, using either airpaths or water paths. In this mode of operation, since the ultrasonicsignal must be reflected from the surface, the characteristics andconfigurations of that surface make for intermittent operation of thesensor. Since the present invention does not depend on signalreflection, the surface configuration becomes of much lesser importanceand reliable, consistent operation is assured.

The location of the surface 17 can be determined from a measurement oftransmission time because the speed of the ultrasonic signal in air andin water differ by a ratio of approximately 1:4. Since the velocity ofsound in air and the velocity of sound in water are known, a simplerelation can be derived which relates transmission time to hull heightor foil depth.

Specifically,

t=(x/c)+x'/c' 1 where t the transmission'time of the ultrasonic signalfrom transmitter 16 to receiver 18, in seconds; x the distance that theultrasonic signal travels along the air path, in feet; 0 the velocity ofsound in air, in feet/second; x the distance that the ultrasonic signaltravels along the water path, in feet; and c the velocity of sound inwater, in feet/second. The velocity of sound in air c is approximately1,100 feet per second, and the velocity c in water is about 4,800 feetper second. Assuming that the total path length x x is feet, relation(1) may be written as follows:

t= (x/l 100) (20x/4800) 2 For an air path distance x of 16 feet, thetransmission time I may be calculated to be 0.01469 second. Furthercalculations indicate that this transmission time would change with hullheight or foil depth at the rate of 0.00089 second/foot. As can be seen,the transmission time t may thus be directly related to either the hullheight or foil depth.

The output signal from transmission time detector 20 may correspond tomeasured signal transmission time, to calculated hull height, or tocalculated foil depth. In the latter cases, detector 20 may includecircuitry which converts the measured transmission times into the heightor depth reading according to the abovementioned relation. Thisconversion may proceed'by either calculation on a continuous basis, orby comparison of the measured transmission times with a precalculatedtable. Alternately, if the output from detector 20 simply corresponds totransmission time, similar conversion circuitry may be provided incontrol system 24 or display 26.

The frequency of the ultrasonic energy source within transmitter 16 isnot material to a consideration of the method of measurement of therelative position of the water surface 17, for the velocities of soundthrough air and through water are relatively constant with respect tofrequency. However, to obtain the necessary resolution between smalldifferences in transmission time, and thus to accurately determine hullheight or foil depth, the frequency of the ultrasonic energy source mustbe considered in conjunction with the particular processing techniqueused within transmission time detector 20. It has been found that forcontinuous wave processing techniques, a frequency of 40 KHz ispreferable while for pulsed processing techniques, a frequency greaterthan 16 KHz is preferable. If continuous wave processing is used with anultrasonic signal of 40 KHz, if the distance x x 20 feet, and if thetransmission time detector 20 is accurate to l percent, the resolutionof position changes is 2 inches.

The prime advantage of the invention is in consistent and reliableoperation during most anticipated water surface configurations withoutthe necessity for a large amount of ultrasonic energy supplied by thetransmitter 16. This advantage can perhaps best be understood bycomparing the propagation loss of an ultrasonic signal through anair-water surface-air, transmission path, as with the sonar systems,with the propagation loss of an ultrasonic signal through an air-watersurface-water, transmission path, as with this invention.

The propagation loss of the ultrasonic signal along the transmissionpath of this invention consists of a loss due to spreading, a loss dueto attenuation, and a loss due to transmission through the watersurface.

The spreading loss occurs for both the air and the water transmissionpaths and results from the fact that all rays of the beam in bothmediums do not travel in parallel. A defined, parallel beam emitted bytransmitter 16 would eliminate the spreading loss in the air medium.However, such a beam could not provide reliable operation of the sensorduring all water surface configurations. As will be explained in greaterdetail, it is desirable to have some spreading of the beam through anangle B to insure transmission of a portion thereof through the water toreceiver 18.

According to a well-known formula, the spreading loss may be defined interms of the path distances x and x as follows:

SL=20 log (x+x')/(3) (3) where SL spreading loss, expressed in db, ordecibels. In this relation, spreading loss is referenced to a soundlevel measured one yard from the source. The attenuation loss isdetermined by multiplying an attenuation constant, a or a, by the pathdistance x or x. The attenuation constant for air, a, is dependent onambient temperature, relative humidity, and frequency of the ultrasonicsignal. For a frequency of 40 KHz, at 15 C. ambient temperature andpercent relative humidity, a 0.1 db/ft. For expected water conditions,

the attenuation constant for water, a, 0.00033 db/ft., which isinsignificant for the short distance x contemplated.

The transmission loss through the water surface, or L is also expressedin db as the following:

L=20log[P, P,] (4) where 1/ 1= (P )2 sin Gil/N Sin 1+( )1 21(5) P, istransmitted pressure (in water);

P is incident pressure (in air);

(PC), is acoustic impedance of first medium (air);

(PC is acoustic impedance of second medium (water);

0, is complement angle of ray path angle of incidence in first medium(air) 0 is complement angle of ray path angle of incidence in secondmedium (water).

In simple terms, the transmission loss is expressed as the ratio of thesound pressure levels of the ultrasonic signal in water and in air. Itshould be cautioned that relation (5) is valid only for planarinterfaces, that is, for a smooth water surface. The value of the lossvaries from this ratio with anomalies in that interface.

Since the acoustic impedance of water is much higher than that of air,that is,

(PC) 155,000 g/s/cm and (PC)1= g/s/cm relation may be reduced to thefollowing:

P lP, [2(PC) sin 0 ]/[(PC) sin 0,] 2 Therefore, from Equation (4)Therefore, because of the widely differingacoustic impedances of thewater and air mediums, there is actually a gain of approximately 6 db inthe transmission of the ultrasonic signal through the water surface.

If the total signal transmission path is again assumed to be feet, andif the length of the water transmission path x is assumed to be 4 feet,then the various components of the propagation loss, or S, may bewritten as follows:

The propagation loss for an air-water surface-air sensor, considering noreflection loss, may be calculated as follows:

As can be seen, the propagation loss for the latter system is about 12db greater than the propagation loss for this invention. This differenceindicates that a lower power level may be utilized to produce theultrasonic signal emitted by transmitter 16.

For a rough water surface, the'6 db gain in sound pressure level throughthe air-water interface is somewhat reduced. However, due to the complexnature of the surface under rough water conditions, the sonar or airpath systems have, in addition to the propagation loss noted in relation(8), an additional reflection loss due to dispersion of the transmittedultrasonic signal in directions other than back to the airlocatedreceiver for such systems. Although the amount of these reflectionlosses cannot be accurately ascertained, the effects thereof are quitenoticeable in rough water, as the operation of the sensor becomesunreliable and inconsistent, no matter what the power level of theultrasonic energy supplied by transmitter 16. By depending for itsoperation upon transmission of the ultrasonic signal through the watersurface 17, this invention is unaffected by reflection losses andtherefore able to provide continuous, consistent, and reliable sensorreadings.

To ensure accurate and reliable operation during all expected ocean waveconditions, the ultrasonic signal emitted by transmitter 16 should havea predetermined beam spread.

Typically, the waves in the ocean have a wavelength to height ratio of7. With such waves, the slope of the surface thereof with respect to thehorizontal is approximately l6. The characteristics of the transmissionpath through the air and the water mediums can thus best be described interms of three water surface conditions: an in-coming wave of 16 slope,the peak or trough of a wave, and an out-going wave of 16 slope. Thesethree situations are illustrated, respectively, in FIGS. 2-4.

In FIG. 2, the in-coming wave 17 has a slope which corresponds to anangle a, 16 with respect to the horizontaLIt is assumed that the beamfrom transmitter 16 is directed -downwardly andvertically. and that theaxis thereof extends along a line which is, of course, inclined at rightangles with respect to the horizontal.

Under these conditions, not all of the beam which is radiated bytransmitter 16 is transmitted through the water surface 17. Thisphenomenon is due-to the fact that a large portion of the beam whichtravels through V the air medium is totally reflected at the surfaceinstead of being refractedand transmitted into the water 'medium. I

The paths of the ultrasonic signals through the air and water mediumsmay be determined from a consideration of the cosine form of Snells law:

c/c'=cos 6/cos 0' (9) where: Y

7 c the velocity of the ultrasonic signal in the air medium, I

0 angle of incidence of ultrasonic signal,

6' angle of refraction of ultrasonic signal, and

c the velocity of the ultrasonic signal in the water medium.

For the sound velocities previously discussed, a critical angle (0exists beyond which ultrasonic signals travelling through air andimpinging on the water surface 17 are totally reflected. For an air andwater path, the critical angle 0 is 76.75.'

Withparticular reference again to FIG. 2, the ultrasonic signal pathsare shown schematically for incoming wave 17. A first ray 33a of thebeam is normal to the surface 17 and passes through with no change indirection as ray 33w. Rays 34a and 35a, respectively, intersect thesurface 17 at the critical angle '0 of 76.75 and therefore are totallyrefracted along the water surface 17. Rays intermediate 34a and 35a,such as ray 32a and ray 33a, comprise the only portion of the beam fromtransmitter 16 that passes through the water surface 17, the remainderbeing totally reflected therefrom.

In the example shown, the entire-water medium is filled with therefracted rays that pass through the surface l7 and a portion of theserays, represented by ray 32w, reach receiver 18 to ensure a continuoustransmission path for the ultrasonic signal.

FIGS. 3 and 4, respectively, illustrate the ultrasonic signal paths forlevel and for out-going waves having a pitch a 16. It will be noted thatin each case only a portion of the beam fromv transmitter 16 isrefracted through the water surface 17, but in each case a continuoustransmission path is maintained between transmitter 16 and receiver 18.

The minimum beam pattern angle [3 required to completely ensonify thewater medium for waves varying from 16 in-coming to 16 out-going pitchcan be calculated as follows. Since axis 30 extends vertically and sincethe angle a, is measured with respect to the horizontal, the anglebetween axis 30 and ray 33a in FIG. 2 is 01,. Likewise, the anglebetween ray 33a and axis 30 in FIG. 4 is equal to 02 Since ray 33a isnormal to surface 17, the angle between ray 33a and 35a in FIG. 2 isequal to Then, the minimum beam pattern angle B from ray 35a in FIG. 2to ray 34a in FIG. 4 can be expressed as follows:

Of course, complete ensonification of the underwater area is desirableonly when thereceiver 18 is physically displaced from the vertical axis30 of the beam from transmitter 16. if, for example, the transmitter 18were placed directly on that axis, the minimum beam pattern angle Bcould be considerably less than 5 8.5.

it can also be noted from FIGS. 2-4 that although the three watersurfaces 17 therein may be of equal height with respect to the foil 14or the hull 10, the transmission time for the ultrasonic signal betweentransmitter 16 and receiver 18 varies because of the differing signal Ipaths during these three water conditions. That is, both the totalsignal path and the individual signal paths vary in length, depending onthe effective portion of the beam from transmitter 16 and the locationon the surface 17 of the region of transmission therethrough. Althoughthese variations would normally cause errors in the detection of eitherhull height or foil depth, they can be compensated for by appropriatecircuitry either within transmission time detector 20, display 26, orthe automatic control system 24. This circuitry could, for example,comprise averaging means which integrates the transmission timesobtained over a relatively short period, or a slope detecting meanswhich indicates whether or not an in-coming or an out-going wave isbeing monitored.

Any method and apparatus for sensing the transmission time of theultrasonic signal from transmitter 16 to receiver 18 that providesdesired resolution of height or depth changes can be used with thisinvention. Two common methods are those utilizing a continuous wavesignal or a pulsed signal. With the former, an ultrasonic signal of agiven frequency is transmitted by transmitter 16. The transmission timeis measured by comparing the phase of the ultrasonic signal received byreceiver 18 with the phase of the transmitted ultrasonic signal. Theadvantage of this method lies primarily in the capability to provide acontinuous monitoring of height or depth changes with no steps in systemresponse. The latter method directly measures the period or time betweenpulse transmission and pulse reception and is particularly advantageousin requiring lower power levels than the continuous wave method.

One apparatus using the continuous wave technique is that known as asing-around circuit. An embodiment of such a circuit is illustrated inFIG. 5. A blocking oscillator 40 is controlled to provide an ultrasonicoutput of a given frequency, say, 40 KHz. This output is applied througha power amplifier 42 to transmitter 16 and is radiated therefrom as theultrasonic signal. The received ultrasonic signal at receiver 18 is fedthrough an amplifier 44 to the input of a gated amplifier 46. The outputof blocking oscillator 40 is also applied to a frequency divider 48whose output is fed back to the control input of oscillator 40 throughgated amplifier 46.

in operation, the gain of the feedback loop including frequency divider48 and gated amplifier 46 is adjusted in response to the difference inphase between the transmitted and received ultrasonic signals.Adjustment of the gain in this manner proportionately varies thefrequency of blocking oscillator 40.

Another output of frequency divider 48 provides pulses whose repetitionrate is proportional to the frequency of blocking oscillator 40. Thesepulses are applied through an amplifier 50 and a frequency doubler 52 tothe input of a discriminator 54. As the pulses applied to discriminator54 are proportional to transmission time, discriminator 54 converts theminto an output signal of a desired type which is likewise proportionalto transmission time. These output signals are applied to the automaticcontrol system 24 and to the display means 26.

While this invention has been described in terms of specific embodimentsof the system elements, it is to be clearly understood by those skilledin the art that the invention is not limited thereto. Rather, thetechnique for sensing either hull height or foil depth has broadapplicability to devices which sense the level of a gaseous liquidinterface relative to a fixed datum point.

Iclaim:

l. A method for detecting the relative position of an interface betweena gaseous medium and a liquid medium, including the steps of:

a. producing an ultrasonic signal at a first point in the gaseousmedium,

b. receiving, at a second point in the liquid medium, the portion ofsaid ultrasonic signal that is trans mitted through said gaseous medium,refracted by said interface, and transmitted through said liquid medium,and

c. measuring the time of transmission of said ultrasonic signal betweensaid first and said second points.

2. The method as recited in claim 1 wherein said ultrasonic signalcomprises a continuous wave, and said step of transmission timemeasurement includes com paring the phase of said received ultrasonicsignal with that of said transmitted ultrasonic.signal.

3. A method as recited in claim 1 wherein said ultrasonic signalcomprises a series of pulses, and said step of transmission timemeasurement includes comparing the time occurrences of said transmittedand said received pulses.

4. A sensor for use with a watercraft which detects the position of asupporting water surface relative thereto, comprising:

a. transmitting means located in the air on the watercraft which directsan ultrasonic signal at the water surface,

. receiving means located below the water surface on the watercraft forreceiving a portion of said ultrasonic signal which is transmittedthrough and refracted by said water surface, and

c. means detecting the time of transmission of said ultrasonic signalfrom said transmitting means to said receiving means.

5. A sensor as recited in claim 4 for use with a hydrofoil watercraftincluding a hull, a hydrofoil, and means supporting the hydrofoil fromthe hull, wherein said transmitting means is located on the hull andsaid receiving means on the hydrofoil, and wherein said transmissiontime detecting means provides an output signal corresponding to hullheight or hydrofoil depth of submergence.

.12. signal from said transmission time detecting means and controllingthe height of the hydrofoil watercraft in response thereto.

9. In combination with a sensor as recited in claim 5, a display meanscoupled to said output signal from said 7 transmission time detectingmeans which provides a visual indication of hull height or hydrofoildepth in response thereto.

1. A method for detecting the relative position of an interface between a gaseous medium and a liquid medium, including the steps of: a. producing an ultrasonic signal at a first point in the gaseous medium, b. receiving, at a second point in the liquid medium, the portion of said ultrasonic signal that is transmitted through said gaseous medium, refracted by said interface, and transmitted through said liquid medium, and c. measuring the time of transmission of said ultrasonic signal between said first and said second points.
 2. The method as recited in claim 1 wherein said ultrasonic signal comprises a continuous wave, and said step of transmission time measurement includes comparing the phase of said received ultrasonic signal with that of said transmitted ultrasonic signal.
 3. A method as recited in claim 1 wherein said ultrasonic signal comprises a series of pulses, and said step of transmission time measurement includes comparing the time occurrences of said transmitted and said received pulses.
 4. A sensor for use with a watercraft which detects the position of a supporting water surface relative thereto, comprising: a. transmitting means located in the air on the watercraft which directs an ultrasonic signal at the water surface, b. receiving means located below the water surface on the watercraft for receiving a portion of said ultrasonic signal which is transmitted through and refracted by said water surface, and c. means detecting the time of transmission of said ultrasonic signal from said transmitting means to said receiving means.
 5. A sensor as recited in claim 4 for use with a hydrofoil watercraft including a hull, a hydrofoil, and means supporting the hydrofoil from the hull, wherein said transmitting means is located on the hull and said receiving means on the hydrofoil, and wherein said transmission time detecting means provides an output signal corresponding to hull height or hydrofoil depth of submergence.
 6. A sensor as recited in claim 4 wherein said ultrasonic signal is directed at the water surface in a vertically extending beam of a minimum beam pattern angle.
 7. A sensor as recited in claim 6 wherein said receiving means is offset with respect to a vertically extending axis of said ultrasonic signal beam and wherein said minimum beam pattern angle equals 58.5* .
 8. In combination with a sensor as recited in claim 5, an automatic control system coupled to said output signal from said transmission time detecting means and controlling the height of the hydrofoil watercraft in response thereto.
 9. In combination with a sensor as recited in claim 5, a display means coupled to said output signal from said transmission time detecting means which provides a visual indication of hull height or hydrofoil depth in response thereto. 